Trouble w/ fuel pump 1955.1

[Gundu7475]

Hey all. I have a mechanical question. I have a 55.1 4400 series dump. I just rebuilt carb and replaced fuel pump. Truck started but fuel pump stopped pumping. I removed it and set it on vise and hooked up a small fuel tank and pumped it. all good. put it back on and nothing…not pumping. It was primed as well. Im at a loss… is there an actuator in there that may not be spinning? What could be causing this?? Thanks so much in advance.